The present invention relates to a method for printing on a can barrel such as a two-piece can. More specifically, the present invention relates to a printing method for printing on a can barrel, ensuring good transferability of bright ink, and providing print having a bright feeling and high image and line reproducibility.
Various printing is applied to the outer surface of a metal can so as to display the contents, an image thereof or a design informing the origin thereof, or to increase its commercial value.
The printed can has a cross-sectional structure, for example, as shown in FIG. 1 (known example), including a metal substrate 1. An undercoat layer 2 called white or size, a printing ink layer 3 and a finishing varnish layer 4 are sequentially provided on the outer surface of the can.
For the printing ink layer, an ink containing a bright pigment such as aluminum flake or fine particulate coated pearl pigment is used so as to impart brightness in some cases.
Also, it is already known to form a can barrel using a metal blank for the can formation after laminating thereon a gravure printed film. For example, JP-A-7-41740 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) describes a polyester film for coating a can material, which is heat-bonded to a metal sheet for a can material via a thermosetting resin-type adhesive to form a protective coating layer. A printed layer comprising a resin composition containing a pearl pigment is provided by gravure printing on one surface of the polyester film and a thermosetting resin-type adhesive is provided on the printed layer.
In the former printed can, the printing ink layer is generally applied to the surface of the can body by an offset system. However, in the case of an ink containing a bright pigment, the dispersion median diameter of the bright pigment in the ink is as small as less than 5 xcexcm. Therefore, the ink layer can have only a low degree of brightness, and the ornamental effect thereby obtained is not satisfactory.
A first reason therefor is that when a bright pigment having a large particle size is used, the bright pigment gathers at the corner of a roller during the transfer of ink and the transfer does not proceed smoothly. Therefore, a bright pigment having a small particle size must be used. Another reason is that the printing ink is kneaded by a roller so as to reduce the median diameter thereof.
In the latter printed can, high brightness may be attained, but the procedure is complicated. Furthermore, when the polyester film is applied to a two-piece can, workability of the film wrap part is troublesome.
The present inventors have carried out extensive investigations to identify the factors which can realize excellent transferability of the bright ink, high brightness and good reproducibility of images and lines even in the case of applying a printing ink containing a bright pigment onto the outer surface of a can by an offset system. As a result, the present inventors found that it is important to select a bright pigment having a relatively large particle size, and to use a engraving roller for picking up the bright ink.
It is therefore a first object of the present invention to provide a printing method for printing on a can barrel using an offset system, which can ensure excellent transferability of bright ink, and which can provide bright print having high image and line reproducibility.
Another object of the present invention is to provide a method for forming a clear and high-quality printed image on the surface of a can barrel without generating transfer failure of the ink, thickening of the image or non-image areas or staining even when a plurality of inks having different properties such as different viscosities are applied.
According to the present invention, a printing method for printing an ink layer on a can barrel is provided, which comprises picking up a bright ink containing at least one bright pigment having an average particle size of from 5 to 25 xcexcm selected from the group consisting of aluminum flake and fine particulate coated pearl pigment from an engraving roller, feeding the picked-up ink to a printing plate directly or via a rubber roller, and applying the ink on the printing plate to a can barrel via a blanket wheel.
In preferred embodiments of the printing method of the present invention,
1. the bright ink has a viscosity of 20 poise or less at a temperature of 35xc2x0 C. and a shear rate of 100 secxe2x88x921;
2. the printing plate is a relief printing plate, and the method comprises forming an ink layer on the can barrel without applying dampening water onto the printing plate (that is, dampening water is not applied to the printing plate during the printing operation);
3. the printing plate has a JISA hardness of 90xc2x0 or less;
4. the rubber roller has a JISA hardness of 60xc2x0 or less;
5. the method comprises feeding the picked-up ink on the engraving roller to the printing plate via a rubber roller, rotating the engraving roller and rubber roller each at respective peripheral speeds, and varying the peripheral speed of the rubber roller relative to that of the engraving roller; and
6. varying the peripheral speed ratio (Vr/Va) within the range of from 0.5 to 1.5, where Vr is the peripheral speed of the rubber roller and Va is the peripheral speed of the engraving roller.
The present invention can be easily applied to multicolor printing. In this case, a multiple roller-type inker is preferably disposed in the periphery of the blanket wheel and pasted inks are fed to a common blanket wheel from the printing plate of the multiple roller-type inker, to thereby perform multicolor printing on a can barrel.